TDMA (Time Division Multiple Access), has the largest network in the US.

TDMA is a digital wireless communication method allowing many users to access a single radio frequency without interference. Every individual user is given a unique time slot within each channel.

Why use TDMA? IT increases the efficiency of transmission and offers a number of other advantages. A major pro is it can be easily adapted to transmission of data as well as voice communication. It offers the ability to carry date rates of 64 kbps to 120 mbps, which enables options of communication such as fax, voiceband data, sms, as well as bandwidth intensive apps. TDMA allows the user to have extended battery life since the mobile is only transmitting a portion of the time during conversations. Furthermore, it is the most cost effective technology for upgrading a current analog system to digital.

How it works:
It’s necessary for TDMA to rely upon that fact that the audio signal has been digitized. These signals are divided into a number of milliseconds. TDMA is also the access technique used in the European digital standard, GSM, and the Japanese digital standard, personal digital cellular (PDC). The reason for choosing TDMA for all these standards was that it enables some vital features for system operation in an advanced cellular or PCS environment. Today, TDMA is an available, well-proven technique in commercial operation in many systems.
A single channel can carry all four conversations if each conversation is divided into relatively short fragments, is assigned a time slot, and is transmitted in synchronized timed.

Downfaults:
TDMA can be wasteful of bandwidth because time slots are allocated to specific conversations whether or not anyone was speaking at the given moment. There is an enhanced version however, EDTMA, which attempts to correct this problem. Unlike TDMA which waits to determine whether a subscriber is transmitting, ETDMA assigns subscribers using a dynamic method. The data is sent through pauses which normal speech contains. If the subscriber has something they would like to transmit, it is placed as one bit in the buffer queue. The system then scans the buffer and notices the user has something to transmit, allocating the bandwidth accordingly. However, if there is nothing to transmit, it goes to the next subscriber. This technique can be 10 times more efficient as analog transmission of TDMA.